More than 150 leaders have gathered for the United Nations climate talks in Paris with rallying calls to take action to secure a new deal to curb global warming.
UK Prime Minister David Cameron, the Prince of Wales and US President Barack Obama were among the leaders to address the conference, at which negotiators from 195 countries will attempt to hammer out a deal to prevent temperatures rising by more than 2C above pre-industrial levels and avoid dangerous climate change.
Mr Cameron called for an agreement that keeps the door open to the 2C goal, urging delegates to think what they would have to say to their grandchildren if they failed to secure agreement.
"Let's imagine what we would have to say to our grandchildren if we failed. We would have to say it was all too difficult. They would reply, well what was so difficult, when the Earth was in peril, when the sea levels were rising in 2015, when crops were failing, when deserts were expanding, what was it that was so difficult," he said.
The Prime Minister set out what was needed to get a "good deal" in Paris: one which had a binding legal mechanism, with five yearly reviews to see how the world was progressing, a deal which provided finance for the poorest and most vulnerable countries, which transferred technology from rich to poor nations and had transparency.
He questioned why it would be difficult to get a legal deal when 75 countries already had legally binding climate change legislation - "countries like Britain, countries that aren't suffering from legally binding legislation, countries that are thriving with that legislation".
He urged delegates at the talks to act now, rather than find excuses tomorrow, saying action on climate change was "not difficult, it's doable".
Environmentalists said the Prime Minister must match his words on the world stage with action back home, after the Government made a series of moves to curb support for renewables, energy efficiency and a £1 billion competition to develop technology to capture and store carbon emissions from power plants.
Greenpeace UK executive director John Sauven said: "The UK's pioneering climate targets and the recent coal phase-out plan show that where Britain leads other countries follow. But we need to see the same UK leadership in the race to develop and invest in renewable technologies.
"This is what Britain's leading businesses, scientists, and the government's own advisers are urging Cameron to do - he should listen."
The Prince of Wales also urged leaders and negotiators gathered in Paris for the start of crucial UN talks to "think of your grandchildren, as I think of mine", as well as the billions of people without a voice and the youngest generation, to try and secure a new deal.
Referring to the delegates as the "few", in echoes of Winston Churchill's description of Battle of Britain fighter pilots, he told them their actions over the next two weeks would determine the fate of present and future generations.
"Rarely in human history have so many people around the world placed their trust in so few. Your deliberations over the next two weeks will decide the fate not only of those alive today, but also of generations yet unborn," he said.
In a key note speech to the conference, Charles said he prayed that in pursuing national interest, countries would not lose sight of international necessity.
Charles, who recently drew links between climate change and the conflict in Syria, said: "Already we are being overtaken by other events and crises that can be seen as greater and more immediate threats.
"But in reality many are already and will increasingly be related to the rapidly growing effects of climate change."
He warned: "On an increasingly crowded planet, humanity faces many threats - but none is greater than climate change.
"It magnifies every hazard and tension of our existence.
"It threatens our ability to feed ourselves; to remain healthy and safe from extreme weather; to manage the natural resources that support our economies and to avert the humanitarian disaster of mass migration and increasing conflict."
But, he said, humanity has the knowledge, the tools and the money to tackle climate change, and lacked only the will and the framework to do so.
US president Barack Obama called the global climate talks an "act of defiance" against terrorism, with the conference taking place just a fortnight after the terror attacks in the French capital which killed 130 people.
"What greater rejection of those who would tear down our world than marshalling our best efforts to save it," Mr Obama said.
As the summit opened, organisers of 2,300 marches and events around the world over the weekend said more than three quarters of a million people had taken part in 175 countries calling for strong action on climate change and a shift to 100% clean energy.